Electrochemical reduction of benzene



United States Patent US. Cl. 204-59 6 Claims ABSTRACT OF THE DISCLOSUREA process for electrochemically reducing benzene to 1,4 cyclohexadienein the presence of liquid ammonia, an alkali metal chloride and analcohol, all in the absence of iron and under anhydrous conditions.

DISCLOSURE This invention relates to a method of electrochemicallyreducing benzene to selectively produce 1,4 cyclohexadiene.

Benzene can be reduced chemically to a mixture of products, such as 1,3and 1,4 cyclohexadienes, cyclohexene, and cyclohexane, in anhydrous lowmolecular weight amines with metallic lithium or sodium. These alkalimetal reductions are of considerable synthetic use, however, the cost oflithium or sodium metal and the amines makes this process too expensivefor large scale use. The products formed by this chemical reduction havesimilar properties and separation into pure components is nearlyimpossible. Aromatic compounds have been reduced electrolytically.Benzene has been reduced in methylamine while using lithium chloride asa current carrier.

It has now been unexpectedly discovered that benzene can be reduced toselectively produce 1,4 cyclohexadiene when said reduction is carriedout electrolytically in the presence of liquid ammonia and an alcoholwith a current carrier of sodium chloride. For highest yields it hasbeen discovered that said reaction should be carried out in the absenceof iron contamination and under anhydrous conditions.

An outstanding feature of this reaction is its selectivity of products.Benzene was converted to 1,4 cyclohexadiene with selectivity of greaterthan 98 percent. The remaining 2 percent was cyclohexene, probablyformed by the reduction of the 1,4 cyclohexadiene. If the reaction wascarried to near completion (reduction of all the benzene), the amount ofcyclohexene would increase; however, the reaction can be carried to 55percent conversion to 1,4 cyclohexadiene with only 2 percent cyclohexenebyproduct and still maintain high current efficiencies. The remainderwas unreacted benzene.

Current efficiencies vary over a wide range and are dependent on manyfactors. Efficiencies of 40 percent can be obtained by using the bestknown conditions and not carrying the reaction past 25 percentconversion. A current efficiency of 30 to 33 percent was obtained wherethe conversion was 55 percent.

The presence of 1 to 4 percent alcohol such as methanol, ethanol, orisopropanol is essential to obtain good current efiiciencies. In theabsence of an alcohol, the current efficiencies are only about 1 to 2percent.

Numerous materials can be used as cathodes. The desirable properties ofthe cathode are: (1) high hydrogen overvoltages, (2) resistance toattack by anhydrous ammonia, (3) good conductor, (4) readily available,and (5) low cost. Aluminum, zinc, and platinum have been used withaluminum being preferred because of its availability and low cost.

Numerous materials were tested as anodes. Included were platinum, gold,titanium, cast iron, stainless steel,

nickel, zirconium, molybdenum, tungsten, antimony, and tin. Good currentefficiencies can be obtained with a platinum anode with 1 percentalcohol present.

The effect of curent density is closely related to the cathode potentialas shown in the table.

TABLE Effect of cathode current density on current efficiency Currentdensity, ma./in. Percent current efficiency Sodium formed at the cathodeby the reduction of sodium ions present will react rapidly with anywater present rather than the benzene. This results in a drop in currentefficiency. Also, iron catalyzes the reaction of sodium with ammonia. Ifiron or other transition metals are present the sodium reacts with theammonia and no reduction of benzene occurs.

The effect of benzene concentration was not as critical as othervariables; however, highest current efficiencies were obtained when thebenzene concentration was 15 to 20 percent.

Temperatures near -33 C. were necessary to obtain good currentefficiencies. The yield was not affected by the temperature.

The over-all reaction for the reduction of benzene to 1,4 cyclohexadieneis:

This is a chemical reaction. The electrode reactions are:

Cathode- 6Na 6e- 6Na Anode 8NH 6e- N +6NH Total electrode reaction- 8NH6Na+ 6Na +N 6NH A reasonable sequence of steps would be the reaction ofsodium with benzene to form a sodium-benzene complex.

3 6Na 3N9. Na

The alcohol then acts as an acid in the ammonia, displaces the sodium,and forms the 1,4 cyclohexadiene.

The alcohol is regenerated by the reaction of the sodium alkoxide withammonium ions which are formed at the anode.

Mass spectroscopy analysis showed the presence of nitrogen and hydrogenin the off-gases. Nitrogen was liberated at the anode from the oxidationof ammonia. Hydrogen was formed by the reaction of sodium with ammonia.

This reaction is catalyzed by the presence of iron and other transitionmetals. The competition between this reaction and the benzene-sodiumreaction thus accounts for the observed 40% current efficiency.

Separation of the products formed can be accomplished by fractionalcrystallization or fractional distillation.

For a fuller understanding of the present invention, reference Will behad to the following example.

EXAMPLE Ten grams of benzene was dissolved in 200 grams of liquidammonia along with 10 grams of sodium chloride. The solution wasmaintained at 20 C. in a stainless steel pressure .vessel lined with apolyethylene liner. The electrodes were strips of platinum with an areaof 25 cm. each. A constant direct current of 20 ma. per cm. was passedthrough the solution for 20 hours. Benzene was reduced at the cathode to1,4 cyclohexadiene. No other reduction products were formed during theelectrolysis. The current eificiency was 25 percent. The remainder ofthe curent Went to form hydrogen gas from the ammonia.

Having thus described the invention by providing a specific examplethereof, it is to be understood that no undue limitations orrestrictions are to be drawn by reason thereof and that many variationsand modifications are within the scope of the invention.

What is claimed is:

1. A process for electrochemically reducing benzene to 1,4cyclohexadiene which comprises:

(a) dissolving benzene and an alkali metal chloride in liquid ammoniaunder substantially anhydrous conditions and in a substantially ironfree vessel containing an anode and a cathode;

(b) passing a direct current of electricity through said solution for aperiod of time sufiicient to produce a mixture of essentially 1,4cyciohexadiene and benzene; and

(c) recovering said 1,4 cyclohexadiene from said mixture.

2. The process of claim 1 comprising the additional steps of addingalcohol to step (a) of claim 1.

3. The process of claim 1 wherein said alkali metal chloride is sodiumchloride.

4. The process of claim 1 wherein the recovery step of (c) is byfractional crystallization.

S. The process of claim 1 wherein the recovery of step of (c) is byfractional distillation.

6. The process of claim 2 wherein the temperature of the reaction ismaintained at about 33 C.

References Cited UNITED STATES PATENTS 3,361,653 1/1968 Miller 204--59HO\VARD S. WILLIAMS, Primary Examiner

